This invention relates to an FM radio data-system receiver for receiving standard broadcast FM audio and FM digital-data signals, and more particularly to such a receiver that re-generates the suppressed data-signal carrier from the received FM audio pilot carrier.
Such radio data systems are currently in commercial use in Europe but in the United States are only beginning to be used. Technical standards to which Europeans work is entitled Specifications of the Radio Data System RDS for VHF/FM sound Broadcasting, Tech. 3244-E, published March 1994 by Technical Centre of the European Broadcasting Union, Avenue Albert Lancaster 32, B-1180 Bruxelles (Belguim). A second draft of the voluntary technical standard in the United States, being developed by a joint committee of the Electronics Industry Association (EIA) and the National Association of Broadcasters (NAB), is entitled United States RBDS Standard-Draft No. 2.0, NRSC Document; Aug. 1, 1992.
According to both standards, digital-data signals are transmitted as a subcarrier as part of conventional broadcast FM signal. The data is carried on a 57 KHz suppressed carrier double sideband subcarrier that is phase synchronous with the 19 KHz pilot carrier. Thus, the FM carrier is modulated at the transmitter with a standard composite stereo multiplexed signal wherein there is added a radio data signal centered about a 57 KHz suppressed carrier as illustrated in FIG. 1.
In the transmitter, the digital-data is differentially clocked by pilot divided by 16, the clock and data then being differentiated by an impulse modulator and subsequently frequency limited to form a spectrally efficient data-symbol modulation signal. These later steps are a means for coding the digital data in Manchester code whereby the resulting data-symbol modulation signal occupies a bandwidth of 4.8 KHz. The data-symbol modulation signal is then fed to a 57 KHz balanced modulator.
With reference to FIG. 2 herein, data recovery in a receiver of the prior art, proceeds by first retrieving the suppressed 57 KHz data carrier from the standard composite stereo multiplexed signal at conductor 14 and demodulating the 57 KHz+/-2.4 KHz band to retrieve the data-symbol signal, typically using a Costas phase locked loop (PLL) illustrated as circuit block 12. Within PLL 12 is a 57 KHz demodulator 13 and a 57 KHz suppressed carrier recovery circuit 15. The 1187.5 Hz clock signal is obtained by using a bit-rate clock recovery circuit 18 that includes a divider (not shown) for dividing the carrier frequency by 48 and a phase error detector (not shown) to correct for the phase error incurred by the frequency dividing.
Demodulator 13 in the PLL 12 provides reconstruction and recovery of the data-symbol signal that is passed through a low pass filter 19. Decoding of the recovered data-symbol signal for display is achieved by a data-symbol decoder 20, a differential decoder 22 and a data processor 24, all with respect to the recovered bit-rate clock signal. Digital-data signal demodulation and retrieval of the 57 KHz using the Costas phase locked loop, in the prior art receiver, is accomplished by first filtering the composite FM-radio stereo signal including the 57 KHz+/-2 KHz data band through a band pass filter 30 for passing only the 4 KHz wide data band. The passed data band signal is then introduced to the Costas phase locked loop (PLL) 12 serving as a synchronous 57 KHz demodulator.
In the prior art receiver, the 57 KHz data signal demodulator operates to retrieve the 57 KHz carrier from the 57 KHz sidebands and its broad band Costas phase locked loop tends to be unstable under noisy conditions. It is therefore essential that the 57 KHz bandpass filter 30 be capable of strongly rejecting the stereo band signals and signals that may be generated by FM radio signals in adjacent FM radio broadcast bands.
The 57 KHz bandpass filter 30 is also required to have rising and fall-off characteristics that are symmetrical about the suppressed 57 KHz carrier so that the phase shifts it imposes on the two side bands of the 57 KHz data-band signal are the same for rendering recovery of the 57 KHz carrier in the demodulator feasible. These stringent performance requirements for the 57 KHz band pass filter 30 leads to the need for many tight tolerance filter components, and in practice such filters have from eight to twelve poles, and are expensive.
When the radio data-system receiver is combined with an FM stereo radio receiver, the receiver front end (i.e. RF tuner, IF amplifier and FM discriminator) produces the composite multiplex signal at a conductor 14 as shown in FIG. 2. From there on, the data recovery and decoding circuits are independent of the stereo pilot retrieval and stereo decoder circuit 36.
A major commercial interest in such FM radio stereo receivers with digital-data reception capability, has been directed to mobile use in automobiles and other vehicles wherein unlike for fixed receiving stations, the received radio signal is subject to drastic changes in signal strength, multipath distortion, and interference. The data to be received by the driver of a vehicle may include current weather conditions, traffic reports and other data of special interest to a traveler.
It is an object of this invention to provide a radio data system receiver with improved stability and reliability of data reception.
It is a further object of this invention to provide such a radio data system receiver having a data system demodulator employing a phase locked loop with a narrow bandpass, obviating the need for a high performance 57 KHz bandpass filter.
It is yet another object of this invention to provide such a radio data system receiver as part of an FM stereo radio receiver, wherein a single narrow band phase locked loop serves both as the stereo decoder and as the primary circuit for regenerating the 57 KHz suppressed data-carrier.